Composition including a lanthanum perovskite on an alumina or aluminium oxyhydroxide substrate, preparation method and use in catalysis

ABSTRACT

The composition according to the invention includes a perovskite of the formula LaMO 3 , where M is at least one element selected from among iron, aluminium or manganese, in the form of particles dispersed on an alumina or aluminium oxyhydroxide substrate, characterized in that after calcination at 700° C. for 4 hours, the perovskite is in the form of a pure crystallographic phase, and in that the size of the perovskite particles does not exceed 15 nm. The composition according to the invention can be used in the field of catalysis.

The present invention relates to a composition comprising alanthanum-based perovskite on a support made of alumina or aluminumoxyhydroxide, to its process of preparation and to its use in catalysis.

Perovskites of general formula ABO₃ are known to exhibit advantageousproperties in the field of catalysis. In this field, they are preferablyused in the supported form, so as to increase the surface area forcontact between the perovskite and the stream to be treated, for examplea gas. The support can in particular be alumina, silica or ceria.

In the case of a supported perovskite, it is important for theperovskite to be dispersed as finely as possible over the support, thatis to say that it is provided in the form of fine particles, ofnanometric sizes, on the support. In addition, as the catalysts areoften exposed to high temperatures, it is also advisable for the finelydivided state of the perovskite to be maintained even at thesetemperatures. In other words, sintering of the perovskite particlesshould not take place.

Furthermore, at high temperature, the appearance of interferingcrystallographic phases other than the pure perovskite phase may also beobserved. The formation of these phases may result in a decrease in thecatalytic activity of the supported perovskite. A search is thusunderway for products capable of retaining a phase purity even at hightemperatures.

The subject matter of the invention is the development of compositionsmeeting these conditions.

To this end, the composition of the invention comprises a perovskite offormula LaMO₃ in which M represents at least one element chosen fromiron, aluminum or manganese, in the form of particles dispersed over asupport based on alumina or aluminum oxyhydroxide, characterized inthat, after calcination at 700° C. for 4 hours, the perovskite exists inthe form of a pure crystallographic phase and in that the perovskiteparticles have a size of at most 15 nm.

The compositions of the invention thus have the advantage of exhibitinga perovskite which is both in the finely dispersed form and in the purecrystallographic form.

In addition, the compositions of the invention can exhibit advantageousreducibility properties.

Other characteristics, details and advantages of the invention willbecome even more fully apparent on reading the description and studyingthe appended drawing, in which:

FIG. 1 is an XRD diffractogram of a product according to the invention.

The term “rare earth metals” is understood to mean the elements from thegroup consisting of yttrium and the elements of the Periodic Table withan atomic number between 57 and 71 inclusive.

For the continuation of the description, the term “specific surface” isunderstood to mean the BET specific surface determined by nitrogenadsorption in accordance with the standard ASTM D 3663-78 drawn up fromthe Brunauer-Emmett-Teller method described in the periodical “TheJournal of the American Chemical Society, 60, 309 (1938)”.

In addition, the calcinations for a given temperature and a given timecorrespond, unless otherwise indicated, to calcinations under air at astationary temperature level for the time indicated.

It is also specified, for the continuation of the description, that,unless otherwise indicated, in all the ranges or limits of values whichare given, the values of the limits are included, the ranges or limitsof values thus defined thus covering any value at least equal to orgreater than the lower limit and/or at most equal to or lower than theupper limit.

The composition of the invention thus comprises a supported perovskitein the form of particles of nanometric sizes, these particles beingdeposited on a support made of alumina.

The perovskite corresponds to the formula LaMO₃ in which M represents atleast one element chosen from iron, aluminum or manganese. The inventionthus definitely covers the case where M can represent a combination oftwo or three of the abovementioned elements.

It is known that the structural stability of perovskites makes possiblethe partial substitution of the cations A and B by cations withidentical or different valencies. For this reason, the invention coversthe cases where at least one of the elements La and M of the perovskiteis partially substituted by at least one substituent element.

Purely by way of example, the substituent element can be chosen fromcalcium and rare earth metals. This rare earth metal can moreparticularly be cerium, yttrium, praseodymium or neodymium.

The substituent element can also be chosen from cobalt and strontium.

Generally, calcium and the rare earth metals are present as substituentfor the element La and cobalt arid strontium as substituent for theelement M but it will be understood that this attribution of a class ofsubstituents to a substituted element is given only by way of example,that it is not absolute and that it is not ruled out for a substituentgiven for one element to be able to substitute for another element.

It may be noted that the combinations of the elements M which werementioned above can be understood as partial substitutions of a firstelement M by a second element M.

The amount of substituent element can vary in a known way in a range ofbetween 1% and 20% approximately, more particularly between 5% and 15%,this amount being expressed by the substituent element/(substituentelement+substituted element) atomic ratio.

The amount of perovskite in the composition can vary within wide limits.This amount can range up to approximately 40% by weight, moreparticularly approximately 35% by weight and more particularly stillapproximately 30% by weight of perovskite, with respect to the totalweight of the composition.

The minimum content of supported perovskite is that from which a personskilled in the art knows that it is possible to obtain a satisfactorycatalytic activity and it is set according to the performance desiredfor the composition. Purely by way of example, this amount of perovskitecan be at least approximately 1% by weight, more particularly at least5% by weight and more particularly still at least 10% by weight. Thisamount of perovskite can thus be comprised between any one of theminimum values given above and any one of the maximum values given abovein the preceding paragraph. More specifically, this amount can thus bebetween 5% and 30% by weight and more particularly between 10% and 20%by weight. It may be noted that, for a given supported perovskite, thesize of the crystallites generally decreases as the amount of perovskitein the composition decreases.

In a known way, the perovskite can exhibit a deficiency or a shortage ofone of the elements La or M. This deficiency can increase the catalyticactivity of the perovskite. This deficiency can be in a range extendingfrom 5% to 30%, more particularly from 10% to 20%, with respect to thestoichiometric amount of the element La or Mn in the perovskite notexperiencing this shortage.

According to a first characteristic of the invention, the perovskiteexists in the form of a pure crystallographic phase even aftercalcination of the composition at 700° C. for 4 hours.

The purity in the crystallographic sense is demonstrated by the X-raydiffraction (XRD) diagram. The diffraction diagram of the composition,after calcination under the conditions indicated, reveals only, inaddition to the crystallographic phase of the alumina or aluminumoxyhydroxide of the support, just the peaks of the perovskite phase. Theappearance is not seen of peaks corresponding, for example, to the oxideLa₂O₃ or to an oxide of the element M.

As indicated above, in the composition of the invention, the perovskiteparticles are deposited on or dispersed over the support. This isunderstood to mean that the perovskite particles are predominantly andpreferably completely present on the surface of this support, it beingunderstood that the particles can be present inside the pores of thesupport but while remaining, however, at the surface of these pores.

According to another characteristic of the invention, these particlesexhibit a size which is at most 15 nm when the composition has beencalcined at 700° C. for 4 hours.

The size values given in the present description are mean sizesdetermined by the XRD technique. The value measured by XRD correspondsto the size of the coherent domain calculated from the width of thethree most intense diffraction lines in the x, y, z space group and byusing the Debye-Scherrer model.

Furthermore, it should be noted that the perovskite particles can eitherbe separate and thus composed of a single crystallite or can optionallybe in the form of aggregates of several crystallites forming a coherentdomain.

According to a preferred embodiment, the particles exhibit a size of atmost 10 nm. In the particular case of the perovskites in which M ismanganese or aluminum, it being possible for these two elementsoptionally to be substituted, sizes of at most 5 nm can be obtained. Thesize values given here are always to be understood for a compositioncalcined at 700° C. for 4 hours.

Finally, it should be noted that the particles can exhibit a very smallminimum size, at the limit of the possibilities of measurement by theXRD technique, for example of the order of 2 to 3 nm, after calcinationat 700° C. for 4 hours.

The support of the composition of the invention can be first of allbased on alumina. Preferably, this support should exhibit a high andstable specific surface, that is to say a specific surface which remainsat a satisfactory value even after exposure to a high temperature.

Use may be made here of any type of alumina capable of exhibiting aspecific surface satisfactory for an application in catalysis. Use maythus in particular be made of an alumina exhibiting a specific surfaceof at least 80 m²/g, preferably of at least 1.00 m²/g, and comprised,for example, between 80 m²/g and 400 m²/g.

Mention may be made of the aluminas resulting from the rapid dehydrationof at least one aluminum hydroxide, such as bayerite, hydrargillite orgibbsite, or nordstrandite, and/or of at least one aluminumoxyhydroxide, such as boehmite, pseudoboehmite and diaspore.

The support can also be based on aluminum oxyhydroxide of theabovementioned type also exhibiting an appropriate specific surface,that is to say as described above concerning the alumina.

According to a specific embodiment of the invention, use is made of astabilized and/or doped alumina or aluminum oxyhydroxide. Mention may bemade, as stabilizing and/or doping element, of rare earth metals,titanium, zirconium and silicon. Mention may very particularly be made,among rare earth metals, of cerium, praseodymium, neodymium, lanthanumor the lanthanum/neodymium mixture. In this instance, lanthanum is thepreferred rare earth metal. These elements can be used alone or incombination.

It should be noted, for the continuation of the description, that theterms “stabilized”, “doped”, “stabilizing” or “doping” should beinterpreted nonlimitingly, it thus being possible for a doping elementto be understood as stabilizing, and vice versa.

The stabilized and/or doped alumina or aluminum oxyhydroxide is preparedin a way known per se, in particular by impregnation of the alumina oraluminum oxyhydroxide with solutions of salts, such as the nitrates, ofthe abovementioned stabilizing and/or doping elements or also bycodrying an alumina or aluminum oxyhydroxide precursor and salts ofthese elements, followed by calcination.

Furthermore, mention may be made of another preparation of thestabilized alumina in which the alumina powder resulting from the rapiddehydration of an aluminum hydroxide or oxyhydroxide is subjected to amaturing operation in the presence of a stabilizing agent composed of alanthanum compound and optionally a neodymium compound, it beingpossible for this compound to be more particularly a salt. The maturingcan be carried out by suspending the alumina in water and then heatingto a temperature of, for example, between 70 and 110° C. After thematuring, the alumina is subjected to a heat treatment.

The content of stabilizing and/or doping element, expressed as weight ofstabilizing oxide with respect to the stabilized and/or doped alumina oraluminum oxyhydroxide, is generally between 1% and 10% approximately.

The compositions of the invention can exhibit perovskite particle sizeswhich still remain low even at temperatures of greater than 700° C.Thus, after calcination at 900° C. for 4 hours, the perovskite particlesexhibit a size of at most 18 nm, more particularly of at most 15 nm. Inthis case, the particles may exhibit a minimum size which can beapproximately at least 5 nm.

After calcination at 1000° C. for 4 hours, the perovskite particlesexhibit a size of at most 22 nm, more particularly of at most 15 nm. Inthis case, the particles may exhibit a minimum size which can beapproximately at least 8 nm.

According to preferred embodiments and more particularly with stabilizedand/or doped alumina or aluminum oxyhydroxide supports, in particularstabilized and/or doped with lanthanum, it is possible to havecompositions in which the perovskite exists in the form of a purecrystallographic phase even after calcination at 900° C. or even at1000° C. for 4 hours.

In addition to the fact that the compositions of the invention comprisea finely dispersed perovskite, some of them exhibit advantageousreducibility properties. They are compositions for which the element Mof the perovskite is iron and/or manganese, it being possible for theseelements to be substituted and it being possible for the perovskite toexperience a shortage. In this case, the supported perovskite exhibits agreater amount of labile oxygen than the same bulk perovskite, forexample at least two times more, indeed even five times more, which isreflected by a higher reductibility of the product.

The process for the preparation of the compositions of the inventionwill now be described.

This process is characterized in that it comprises the following stages:

-   -   a liquid medium is formed which comprises alumina or aluminum        oxyhydroxide and salts of the elements La and M and, if        appropriate, of a substituent element, said salts being chosen        from acetates, chlorides and nitrates;    -   a base is added to the medium thus formed until a pH of at least        9 is obtained, whereby a precipitate is obtained;    -   the precipitate is separated from the reaction medium and, in        the case of the use in the first stage of chlorides or nitrates        as salts of the abovementioned elements, the precipitate is        washed;    -   the precipitate is calcined.

The first stage of the process thus consists in forming a liquid medium,generally an aqueous medium, which comprises, in the form of adispersion, alumina or aluminium oxyhydroxide which will act as supportin the composition which it is desired to prepare.

According to a preferred alternative form, the alumina or aluminumoxyhydroxide may have been precalcined, for example at a temperaturewhich can be between 500° C. and 700° C., in order to avoid anexcessively great variation in the crystallographic characteristics inthe continuation of the preparation process.

This liquid starting medium additionally comprises salts of the elementsLa and M and, in the case of the preparation of compositions in whichthe elements La and M of the perovskite are substituted, the salts ofthe substituent elements.

The salts are chosen from the acetates, chlorides and nitrates. Theacetates are preferably used if it is desired to obtain the smallestpossible crystallite sizes. The chlorides may exhibit the advantage ofmore easily resulting in compositions in which the perovskite remains inthe form of a pure crystallographic phase at 900° C. or 1000° C.

In a second stage of the process of the invention, a base is added tothe medium formed in the preceding stage.

Use may be made, as base, of products of the hydroxide, carbonate orbasic carbonate type, for example. Mention may be made of alkali metalor alkaline earth metal hydroxides or secondary, tertiary or quaternaryamines. However, amines and ammonia may be preferred insofar as theyreduce the risks of contamination by alkali metal or alkaline earthmetal cations. Mention may also be made of urea.

The base is added until a pH of the reaction medium of at least 9, moreparticularly of at least 9.5, is obtained.

The operation of bringing into contact with the base results in theformation of a precipitate in suspension in the liquid reaction medium.

The base is preferably added with stirring. The stirring may bemaintained after this addition, for example for a time of at least onehour.

On conclusion of this second stage, the precipitate is separated fromthe liquid medium by any known means.

The precipitate thus obtained can be washed, in particular with purewater or aqueous ammonia solution. It should be noted that washing isnot necessary in the case of the use of acetates as starting salts; onthe other hand, it is required in the case of the use of chlorides ornitrates, it being possible for the absence of washing in this case toresult in compositions in which the perovskite phase is not pure.

The final stage of the process is a calcination stage.

This calcination makes it possible to develop the crystallinity of thesupported perovskite and it can also be adjusted and/or chosen as afunction of the subsequent operating temperature intended for thecomposition according to the invention, this being done while takinginto account the fact that the specific surface of the product decreasesas the calcination temperature employed increases. Such a calcination isgenerally carried out under air but a calcination carried out, forexample, under an inert gas or under a controlled atmosphere (oxidizingor reducing) is very clearly not ruled out.

In practice, the calcination temperature is generally limited to a rangeof values between 500° C. and 800° C., preferably between 600° C. and700° C. The calcination time is adjusted in a known way; it can vary,for example, between 30 minutes and 4 hours, this time generallydecreasing as the temperature increases.

The compositions of the invention as described above or as obtained bythe process described above are provided in the form of powders but theycan optionally be shaped in order to be provided in the form ofgranules, beads, cylinders or honeycombs of variable dimensions.

The compositions of the invention described above or obtained by theprocess described above can be used as catalysts. Thus, the inventionalso relates to catalytic systems comprising these compositions. Thesesystems comprise a coating (wash coat), based on these compositions andoptionally on a binder of known type and having catalytic properties, ona substrate of the, for example, metal or ceramic monolith type. Thiscoating is obtained by mixing the composition with the binder so as toform a suspension which can subsequently be deposited on the substrate.

These catalytic systems and more particularly the compositions of theinvention can have a great many applications. They are thus particularlywell suited to, and thus usable in, the catalysis of various reactions,such as, for example, dehydration, hydro-sulfurization,hydrodenitrification, desulfurization, hydrodesulfurization,dehydrohalogenation, reforming, steam reforming, cracking,hydrocracking, hydrogenation, dehydrogenation, isomerization,dismutation, oxychlorination, dehydrocyclization of hydrocarbons orother organic compounds, oxidation and/or reduction reactions, the Clausreaction, the treatment of exhaust gases from internal combustionengines, in particular in automobile after burning and in particularthree-way catalysis, demetallation, methanation, the shift conversion orthe catalytic oxidation of the soot emitted by internal combustionengines, such as diesel engines or gasoline engines operating under leanburn conditions. Finally, the catalytic systems and the compositions ofthe invention can be used as catalysts for the selective reduction ofNO, by a reaction for the reduction of these NO₂ by any reducing agentof the hydrocarbon type or also by ammonia or urea and, in this case, ascatalysts for the reaction for the hydrolysis or decomposition of ureato give ammonia (SCR process).

Finally, the compositions of the invention can also be used intreatments for the purification of the air in the case of an aircomprising at least one compound of the carbon monoxide, ethylene,aldehyde, amine, mercaptan or ozone type and generally of the type ofthe volatile organic compounds or atmospheric contaminants, such asfatty acids, hydrocarbons, in particular aromatic hydrocarbons, andnitrogen oxides (for the oxidation of NO to NO₂), and of the malodorouscompounds type. Mention may more particularly be made, as compounds ofthis sort, of ethanediol, valeric acid and trimethylamine.

In the case of these uses in catalysis, the compositions of theinvention can be employed in combination with precious metals or alsotransition metals in the oxide, sulfide or other form and they thus actas support for these metals. The nature of these metals and thetechniques for incorporating the latter in the support compositions arewell known to a person skilled in the art. For example, the metals canbe gold, silver, platinum, rhodium, palladium or iridium, molybdenum,tungsten, nickel, cobalt, manganese, copper, titanium or vanadium; theycan be used alone or in combination and they can in particular beincorporated in the compositions by impregnation.

For the treatment of exhaust gases, the abovementioned systems arefitted in a known way in the exhaust mufflers of motor vehicles.

Examples will now be given.

COMPARATIVE EXAMPLE 1

This comparative example relates to the preparation of an unsupportedbulk perovskite based on lanthanum and iron (LaFeO₃).

For this, 20.32 g of iron nitrate, 30.24 g of a 2.785 mol·l⁻¹ lanthanumnitrate solution and 63.69 g of citric acid are mixed. 20 ml of waterare instantaneously added and mixing is carried out at ambienttemperature for 20 minutes. Heating is carried out until a gel isobtained, which gel is subsequently placed in an oven at 120° C. for 12hours.

The solid obtained is ground and then calcined under air at 700° C. for2 hours.

EXAMPLE 1

This example relates to the preparation according to the process of theinvention of a composition based on a lanthanum iron perovskite (LaFeO₃)dispersed over a doped alumina support comprising 6% of lanthanum in therespective proportions of perovskite and support, as weight of oxide, of10% and 90%.

A solution comprising lanthanum-doped alumina and iron acetate andlanthanum acetate salts is prepared beforehand. For this, 2.26 g of ironacetate are diluted in 75 ml of water and 4.46 g of lanthanum acetateare diluted in 75 ml of water. These solutions are mixed and the mixturethus obtained exhibits a pH of 5.4. Moreover, a dispersion comprising 27g of doped alumina, precalcined at 700° C. for 2 hours under air, with aspecific surface of 180 m/g, dispersed in 150 ml of water is prepared.The solution of iron acetate and of lanthanum acetate is added to thealumina dispersion in order to obtain a liquid mixture comprisinglanthanum-doped alumina and iron acetate and lanthanum acetate salts.11.3 g of a 28% NH₄OH solution are added to this mixture so that thefinal pH reaches 10; the formation of a precipitate is observed. Theprecipitate obtained is kept stirred at ambient temperature for 1 h 30.This precipitate is filtered off on a Büchner funnel.

The powder obtained is calcined under air at 700° C. for 4 hours.

FIG. 1 is the XRD diffractogram produced from the powder thus obtained.This FIGURE reveals a pure perovskite phase.

EXAMPLE 2

This example relates to the preparation according to the process of theinvention of a composition based on a lanthanum iron perovskite which issubstoichiometric in lanthanum (La_(0.9)FeO₃) and which is dispersedover the same alumina support as that of example 1 and in the respectiveproportions of perovskite and of support, as weight of oxide, of 10% and90%.

2.4 g of iron acetate are diluted in 75 ml of water and 4.25 g oflanthanum acetate are diluted in 75 ml of water. These solutions aremixed and the mixture thus obtained exhibits a pH of 5.5. Moreover, adispersion comprising the same amount of alumina of example 1 isprepared. The solution of iron acetate and of lanthanum acetate is addedto the alumina dispersion. 10 g of sodium hydroxide are subsequentlyadded to the mixture thus obtained, so that the final pH reaches 10 andthe formation of a precipitate is observed. The precipitate obtained iskept stirred for 1 h 30. This precipitate is filtered off on a Büchnerfunnel.

The powder obtained is calcined under air at 700° C. for 4 hours.

The XRD diagram produced from the powder thus obtained reveals a pureperovskite phase.

EXAMPLE 3

This example relates to the preparation according to the process of theinvention of a composition based on a lanthanum iron perovskite dopedwith cerium and with calcium (La_(0.9)Ce_(0.05)Ca_(0.05)FeO₃), dispersedon the same alumina support as that of example 1 and in respectiveproportions of perovskite and of support, as weight of oxide, of 10% and90%.

On the one hand, 4.09 g of lanthanum acetate, 0.22 g of cerium acetateand 0.11 g of calcium acetate are diluted in 75 ml of water and, on theother hand, 2.31 g of iron acetate are diluted in 75 ml of water. Thesesolutions are mixed in order to obtain an acetate mixture at a pH of5.5. Moreover, a dispersion comprising the same amount of alumina ofexample 1 is prepared. The solution of iron acetate, cerium acetate,calcium acetate and lanthanum acetate is added to the aluminadispersion. 14 g of sodium hydroxide are subsequently added to themixture thus obtained, so that the final pH reaches 10 and the formationof a precipitate is observed. The precipitate obtained is kept stirredfor 1 h 30. This precipitate is filtered off on a Büchner funnel.

The powder obtained is calcined under air at 700° C. for 4 hours.

The XRD diagram produced from the powder thus obtained reveals a pureperovskite phase.

EXAMPLE 4

This example relates to the preparation according to the process of theinvention of a composition based on a lanthanum iron perovskite (LaFeO₃)dispersed over an alumina support in the respective proportions ofperovskite and of support, as weight of oxide, of 10% and 90%.

1.36 g of iron chloride are diluted in 50 ml of water and 6.17 g oflanthanum chloride are diluted in 50 ml of water. These solutions aremixed and the mixture thus obtained exhibits a pH of 2. Moreover, adispersion comprising 18 g of alumina (obtained from a boehmite calcinedat 700° C. for 2 hours) dispersed in 100 ml of water is prepared. Thesolution of iron chloride and lanthanum chloride is added to the aluminadispersion. 9.9 g of a 28% NH₄OH solution are added to the mixture thusobtained, so that the final pH reaches 10; the formation of aprecipitate is observed. The precipitate obtained is kept stirred for 1h 30. This precipitate is filtered off on a Büchner funnel and washed 8times at the same volume.

The powder obtained is calcined under air at 700° C. for 4 hours.

The XRD diagram produced from the powder thus obtained reveals a pureperovskite phase.

The XRD diagram produced from this same powder but calcined at 900° C.for 4 hours or calcined at 1000° C. for 4 hours reveals a pureperovskite phase in both cases.

EXAMPLE 5

This example relates to the preparation of the same composition as thatof example 4 but by a process employing nitrates.

3.35 g of iron nitrate are diluted in 50 ml of water and 4.98 g oflanthanum nitrate are diluted in 50 ml of water. 9.7 g of a 28% NH₄OHsolution are added to the same alumina dispersion as that of example 4,so that the final pH reaches 10; the formation of a precipitate isobserved. The precipitate obtained is kept stirred for 1 h 30. Thisprecipitate is filtered off on a Büchner funnel and washed 7 times atthe same volume.

The powder obtained is calcined under air at 700° C. for 4 hours.

The XRD diagram produced from the powder thus obtained reveals a pureperovskite phase.

EXAMPLE 6

This example relates to the preparation according to the process of theinvention of a composition based on a lanthanum manganese perovskite(LaMnO₃) dispersed over an alumina support in the respective proportionsof perovskite and of support, as weight of oxide, of 10% and 90%.

3.04 g of manganese acetate are diluted in 75 ml of water and 4.47 g oflanthanum acetate are diluted in 75 ml of water. These solutions aremixed and the mixture thus obtained exhibits a pH of 7.2. Furthermore, adispersion comprising 27 g of alumina, precalcined under air at 700° C.for 2 hours, dispersed in 150 ml of water is prepared. The mixture ofmanganese acetate and lanthanum acetate is added to the aluminadispersion. 9.5 g of a 28% NH₄OH solution are subsequently added to themedium thus obtained, so that the final pH reaches 10; the formation ofa precipitate is observed. The precipitate obtained is kept stirred atambient temperature for 30 minutes. This precipitate is filtered off ona Büchner funnel.

The powder obtained is calcined under air at 700° C. for 4 hours.

The XRD diagram produced from the powder thus obtained reveals a pureperovskite phase.

EXAMPLE 7

This example relates to the preparation according to the process of theinvention of a composition based on the same perovskite and on the samesupport as those of example 6 but in the respective proportions ofperovskite and of support, as weight of oxide, of 20% and 80%.

6.08 g of manganese acetate are diluted in 75 ml of water and 8.94 g oflanthanum acetate are diluted in 75 ml of water. These solutions aremixed and the mixture thus obtained exhibits a pH of 7.2. This mixtureis added to the same alumina dispersion as that of example 6. 9.5 g of a28% NH₄OH solution are subsequently added to the medium thus obtained,so that the final pH reaches 10; the formation of a precipitate isobserved. The precipitate obtained is kept stirred at ambienttemperature for 60 minutes. This precipitate is filtered off on aBüchner funnel.

The powder obtained is calcined under air at 700° C. for 4 hours.

The XRD diagram produced from the powder thus obtained reveals a pureperovskite phase.

The sizes of the perovskite particles in the compositions of each of theexamples according to the invention, after calcination of thecompositions at the temperatures indicated for 4 hours, are given intable 1 below.

TABLE 1 Sizes of the particles (nm) as a function of the temperatureExample 700° C. 900° C. 1000° C. 1 8 9 12 2 9 11 12 3 9 11 12 4 12 14 205 8 13 20 6 3 8 30 7 10 16 20

The results of the measurements of reducibility of the compositionsaccording to the comparative examples and according to the invention aregiven in table 2 below.

The reducibility is measured by temperature-programmed reduction in thefollowing way.

Use is made of a Micromeritics Autochem 2920 device with a quartzreactor and a 200 mg sample of product which has been precalcined underair at 700° C. for 6 hours.

The gas is hydrogen at 10% by volume in argon and with a flow rate of 25ml/min. The temperature rise takes place from ambient temperature to900° C. at the rate of 20° C./min. The signal is detected with a thermalconductivity detector. The temperature is measured in the sample using athermocouple.

The hydrogen consumption is calculated from the missing area of thehydrogen signal from the baseline at 30° C. to the baseline at 900° C.This hydrogen consumption, with respect to the weight of the perovskitephase, which characterizes the reducibility properties of the productstested, is given in table 2.

TABLE 2 Example H₂ volume in ml/g of perovskite 1 47.6 2 49.4 3 53.2 444.9 5 65.1 6 51.8 7 62.0 Comparative 1 8.5

It is apparent that the supported perovskite of the compositions of theinvention indeed exhibits markedly improved reducing properties, itbeing possible for the reducibility thus to be 5 times greater in thecase of example 1, with respect to comparative example 1, forperovskites with the same composition.

1. A calcined composition comprising: a perovskite of formula LaMO₃ inwhich M represents at least one element chosen from iron, aluminum ormanganese in the form of particles, wherein at least one of the elementsLa and M of the perovskite is optionally substituted with a substituentelement, and a support based on alumina or aluminum oxyhydroxideprecalcined at a temperature between greater than 500° C. and less than700° C., wherein the perovskite particles are dispersed over thesupport, and wherein, after calcination at 700° C. for 4 hours, theperovskite exists in the form of a pure crystallographic phase and theperovskite particles have a size of at most 15 nm.
 2. The composition asclaimed in claim 1, wherein the particles exhibit a size of at most 10nm.
 3. The composition as claimed in claim 1, wherein the amount ofperovskite in the composition is between 5% and 30% by weight.
 4. Thecomposition as claimed in claim 1, wherein at least one of the elementsLa and M of the perovskite is partially substituted by at least onesubstituent element.
 5. The composition as claimed in claim 4, whereinthe substituent element for La is chosen from rare earth metals andcalcium and the substituent element for M is chosen from cobalt orstrontium.
 6. The composition as claimed in claim 1, wherein theperovskite exhibits a deficiency of one of the elements La or M.
 7. Thecomposition as claimed in claim 1, wherein, after calcination at 900° C.for 4 hours, the perovskite particles exhibit a size of at most 18 nm.8. The composition as claimed in claim 1, wherein, after calcination at1000° C. for 4 hours, the perovskite particles exhibit a size of at most22 nm.
 9. The composition as claimed in claim 7, wherein the perovskiteexists in the form of a pure crystallographic phase.
 10. The compositionas claimed in claim 1, wherein the alumina or aluminum oxyhydroxide ofthe support is stabilized and/or doped with at least one stabilizingelement chosen from rare earth metals, titanium, zirconium and silicon.11. (canceled)
 12. A catalytic system comprising, composition as claimedin claim
 1. 13. The composition as claimed in claim 3, wherein theamount of perovskite is between 10% and 20% by weight.
 14. Thecomposition as claimed in claim 7, wherein the perovskite particlesexhibit a size of at most 15 nm.
 15. The composition as claimed in claim14, wherein the perovskite exists in the form of a pure crystallographicphase.
 16. The composition as claimed in claim 8, wherein the perovskiteexists in the form of a pure crystallographic phase.